1. Field of the Invention
This invention pertains generally to controlling the operation of a continuously variable transmission, and more particularly to a method and system for controlling, for example, the clamping and differential pressures in a continuously variable transmission to achieve a desired rate of change of ratio.
2. Description of Related Art
This application incorporates by reference U.S. Pat. Nos. 6,116,363, 6,054,844, 5,842,534, PCT International Publication No. WO 00/25417, PCT International Publication No. WO 02/058209 A1, and PCT International Publication No. WO 00/12918, each of which is related to this application.
The concept of an engine and a “continuously variable transmission” is a very old concept invented in the 1900's, but the theoretical efficiency of the engine, performance and drivability could never be obtained automatically. This can be seen with reference to the conventional powertrain and transmission shown in FIG. 1 where an internal combustion engine 10 has an output shaft 12 that drives a decoupling/starting clutch or torque converter 14, which is in turn coupled to the input shaft 16 of a continuously variable transmission (CVT) or automatic transmission (AT) 18, which in turn has an -output driving a drive shaft or differential 20 coupled to a final drive wheel 22 (e.g., axle and tire). The deficiencies of such a configuration are caused by the dynamic equation representing the engine/CVT system:
            α      DS        =                                        -                          R              ∘                                ⁢                      I            E                    ⁢                      S            E                          +                              T            E                    ⁢          R                -                  T          loss                -                  T          RL                                      I          DS                +                              R            2                    ⁢                      I            E                                ,            R      ∘        =                  ⅆ        R                    ⅆ        t            where αDS=acceleration of the vehicle reflected to the drive shaft,
      R    =                  S        E                    S        DS              ,IE=engine inertia, IDS=vehicle inertia at the driveshaft, SE=engine speed, SDS=drive shaft speed, TE=engine torque, Tloss torque losses, and TRL=road load torque at the driveshaft. Because the first term—IESE and the second term TER generally oppose each other, the acceleration of the car and the torque and speed of the engine are difficult to control simultaneously. As a result, the best efficiency and minimum emissions for a gasoline or diesel engine cannot be realized without a sacrifice in performance. This can be seen with further reference to FIG. 2 and FIG. 3 which show operating characteristics of the engine as a function of engine speed and torque, where WOT=wide open throttle and denotes the maximum torque line, IOL=ideal torque/speed operating line and denotes where the best efficiency and/or least emissions (minimum brake specific fuel consumption or BSFC) occurs, and POL=practical operating line due to engine/transmission characteristics. Note in FIG. 3 that point A is less efficient than point B but must be used to provide proper vehicle behavior (transient performance).
As discussed in PCT International Publication No. WO 00/25417, the foregoing deficiencies can be overcome, for example, by inserting an electric motor or motor/generator, a battery, and associated controls between the engine and the continuously variable or automatic transmission. More particularly, a motor/generator is controlled to counteract the negative effect of the—IESE in the dynamic equation. The motor/generator can then be used to allow the engine to operate at “wide open throttle” (WOT), or along the “Ideal Torque/Speed Operating Line” (IOL) for best efficiency and lowest emissions, or along any other predetermined operation line. In this way, the engine can be run continuously while energy flows into or out of the battery energy storage system connected to the electric motor/generator. If the battery is large enough to drive the vehicle a long distance, then the efficiency of energy into and out of the battery is high since the battery internal resistance is low. The emissions of the gasoline or diesel engine can be controlled effectively because the engine is operated at high load consistently. This approach ensures that the gasoline or diesel engine is never operated at closed throttle at high speeds or operated at low efficiency low load conditions. If the power required is lower than the minimum power of the engine on the IOL, the engine is automatically decoupled and stopped (or idled if desired), and the vehicle is operated as an electric vehicle.
More particularly, FIG. 4 shows an electric motor 24 coupled to the input shaft 16 of the continuously variable transmission 18 so that it injects power in parallel with the drive train between engine 10 and continuously variable transmission 18. Electric motor 24 is powered by a battery 26, which would typically comprise a bank of batteries, ultra-capacitors or the like, such as those used in electric vehicles. Operation of electric motor 24 is controlled by a motor controller 28, which is a conventional electronic armature controller or the like, which is in turn controlled by a microprocessor or other computer-based programmable system controller 30.
System controller 30 processes a plurality of control and feedback signals. As shown, the primary input control signals are from the vehicle accelerator pedal 32 and brake pedal 34. Based on these signals, system controller 30 sends a throttle control signal 36 to engine 10 to control the engine torque TE, an engine engagement on/off signal 38 to clutch 14, a torque control signal 42 to motor controller 28 to control motor torque TM, and a rate of change of speed ratio control signal 44 to control the rate of change  of the speed ratio R of continuously variable transmission 18, where
      R    =                  S        E                    S        DS              ,SE=engine speed and SDS=driveshaft speed. It should be noted thatSDS=SCAR×C where SCAR is the speed of the vehicle and C is a constant dependent on the gear ratio of the final drive and tire radius for the vehicle. At the same time, system controller 30 senses engine speed SE via speed signals 40, the ratio R via signals 46, and vehicle speed SCAR via signals 48. Note that the system controller 30 may send an “on/off” signal to engine 10, but a separate starter motor is not needed; electric motor 24 can be used start engine 10 because it is coupled to engine output shaft 12 through clutch 14. The engine 10 may be turned “off” or idled when clutch 14 is opened.
Referring to FIG. 5, it will also be appreciated that the foregoing techniques can be extended to a series hybrid vehicle configuration as shown in which a generator 50 is used to provide charging capability for battery 26 as well as to provide a braking effect for engine 10 during deceleration. Operation of generator 50 is preferably controlled by a generator controller 52, which is a conventional electronic armature controller or the like. Generator controller 52 controls generator torque, TG, in response to signals received from system controller 30 through torque control line 54. Note that TG=TE in this configuration. Note also the inclusion of an optional starter control line 56 for starting and shutting down engine 10.
Note that operation of the engine in the above configuration is considerably different than in a series hybrid vehicle where the engine is always running at one speed. When the engine is operated at a constant speed, the efficient power output only occurs at one level. Thus the batteries will have to absorb excess power or provide additional power to drive the vehicle. This results in considerable deep battery cycling and attendant inefficiencies. In the systems shown in FIG. 4 and FIG. 5, the engine is used more and the batteries are shallow cycled. Because the amount of power cycled by the batteries is greatly reduced with the present invention, the range per battery charge is increased. Battery life is increased as well.
Referring now to FIG. 4, FIG. 6, and FIG. 7 together, system controller 30 implements the control and sensing functions of the system using conventional hardware and/or software. In FIG. 6, AC=accelerator pedal position and represents power or torque commanded by the driver (PC or +TC, respectively); BC=brake pedal position representing negative torque commanded by the driver (−TC); TM=electric motor torque; PEP=the error or difference between the power commanded by the driver and the power along the IOL for the power control mode (PC−PIOL); TEP=the error or difference between the torque commanded by the driver and the torque along the IOL for the torque control mode
      (                  T        C            -                        P          IOL                          S          E                      )    ;PIOLE=the power along the ideal operating line of the engine; PIOLM=the power along the ideal operating line of the electric motor; IRL=the ideal regeneration line for braking; TEB=the error or difference between the braking commanded by the driver and the braking along the IRL for the braking control mode (BC−TIRL); TIRL=the torque along the ideal regeneration line for braking; K1=a gain adjustment for desired response time and stability of the circuit, K2=a gain adjustment set in response to SE  in order to achieve the desired response characteristics in FIG. 7, T=the time constant of the filter, S=the Laplace transform of variable PEP or TE which is easily programmed by those skilled in the art; R=the ratio between engine speed and driveshaft speed; =the rate of change of ratio R; C=a conversion constant to convert vehicle speed to driveshaft speed; SE=engine speed; SOS=drive shaft speed; SCAR=vehicle speed; and KB is a gain value for scaling. When the accelerator pedal is depressed, switches SW1 and SW2 go to the accelerator position. Switches SW3 and SW4 will be set according to whether the vehicle is in the electric or hybrid mode. Similarly, when the brake pedal is depressed, switches SW1 and SW2 go to the brake position. Each of these switches generally may be software switches in system controller 30. The IOLE of the engine is obtained by testing the engine to determine the best efficiency and emissions at each speed. The IOLM and IRL are obtained by testing the electric motor/generator and battery system to obtain the most energy into the battery at each speed. Note that the IOLM is used when the vehicle is in the electric drive mode where the vehicle is operated, generally, below freeway speeds until the batteries are depleted to a predetermined state as, for example, described in U.S. Pat. No. 5,842,534.
There are also many possible control algorithms for hybrid electric vehicles. The control objective in the above example is to drive the vehicle using electric energy until the internal combustion engine is turned “on” and then to drive the vehicle with the internal combustion engine as much as possible, automatically supplementing the internal combustion engine with electric energy when needed to maintain operation of the engine along the IOL. Significantly, energy may be put back into the batteries temporarily when the engine power is reduced in order to keep the engine on the IOL at all times in the hybrid mode. This kind of operation can significantly reduce emissions and increase engine efficiency.
In operation, system controller 30 senses the acceleration command AC from the accelerator pedal and the switches SW1 and SW2 shown in FIG. 6 go to the accelerator position. When power or a positive torque is commanded by the driver (PC or +TC) in the electric vehicle mode determined by SW3 and SW4 as the case may be depending upon whether or not the system is operating in the power control region or the torque control region shown in FIG. 7, the system is in an acceleration mode and the desired motor torque TM is then determined at 114 according to
      T    M    =                              P          C                          S          E                    +                        K          2                ⁢                  S          E                ⁢                  R          ∘                ⁢                                  ⁢        or        ⁢                                  ⁢                  T          M                      =                  T        C            +                        K          2                ⁢                  S          E                ⁢                  R          ∘                    If the vehicle is in the hybrid-mode, then TM is determined at 126 according to
      T    M    =                              P          C                          S          E                    -              T                  IOL          E                    +                        K          2                ⁢                  S          E                ⁢                  R          ∘                ⁢                                  ⁢        or        ⁢                                  ⁢                  T          M                      =                  T        C            -              T                  IOL          E                    +                        K          2                ⁢                  S          E                ⁢                  R          ∘                    The motor torque signal determined above is sent to motor controller 28 in FIG. 4 to vary the speed and power of engine 10 and to drive the car. The resultant change in electric motor torque in turn affects the vehicle dynamics at 102, which affect engine speed, vehicle speed and the ratio R at CVT 18. Taking the speed of the vehicle SCAR as well as the ratio R at 102, in FIG. 6, engine speed SE (which may also be the same as the motor speed SM where they are on a common shaft) can be determined by applying a conversion constant C to the vehicle speed SCAR at 104 to get the speed SDS of driveshaft 20 of FIG. 4 (which is the output of CVT 18) and then multiplying the driveshaft speed SDS by the ratio R at 106 in FIG. 6 to give the engine speed SE. Now having engine speed SE, at 108, 116 and 128 look-up tables containing the IOL entries for the hybrid mode, braking mode and the electric mode, respectively, are accessed to determine the ideal engine power or torque output level for the given speed. Then, at 110 for the hybrid mode, 118 for the braking mode or 130 for the electric mode, the output of the corresponding look-up table is compared with either the power PC (if in power control mode) or positive torque+TC (if in torque control mode) commanded by the driver with the accelerator pedal as sensed from accelerator pedal position AC to determine a power error PEP or a torque error TEP. The corresponding error signal is then used to affect the rate of change  of the ratio R after filtering the signal at 112. CVT 18 of FIG. 4 thus responds in accordance with the adjustment of the rate of change of ratio, .
Note that an important aspect of the control system is the control of the rate of change of the ratio R; that is, the control of . This is accomplished by filtering the error signal between the commanded power PC or torque TC and the IOL power or torque. The signal filtering, which is in the form of
      K    1    ·      1          TS      +      1      is well known in the art of electrical engineering. It is understood that this filter is only representative of one form that may be placed at this point, and in practice the filter may include both linear and non-linear elements. The purpose of the filter is to allow the designer to control the ratio rate, . It is undesirable to change R quickly and, therefore, a filter is necessary to provide the desired system response. The values of K1 and T are heuristically determined, as is the form of the filter (which is shown here as first order). Those skilled in the art will appreciate that filters of many other representations will work and can be selected depending on the desired response.
During braking, torque is being commanded at the wheels rather than engine power. Here, system controller 30 senses the braking command BC from the brake pedal. When the driver commands negative torque −TC, the system is in a deceleration (regeneration) mode and the switches go to the brake position. Here, control of the CVT and electric motor/generator reverses to produce a negative torque on the driveshaft, thus braking the vehicle. The operation of the braking circuit is similar to that of the accelerator circuit except for the use of the ideal regeneration line IRL, which reflects the highest efficiency for a given power for regenerating energy into the batteries by the electric motor/generator.
For purposes of braking, the desired motor torque TM is determined at 100 according to
      T    M    =                    T        C            R        -                  K        2            ⁢              S        E            ⁢              R        ∘            and the signal is sent to motor/generator controller 28 to vary the speed and power of engine 10. The resultant change in electric motor/generator and engine torque again affect the vehicle dynamics at 102, to slow the car which affects motor and/or engine speed, vehicle deceleration and the ratio R at CVT 18. Here, however, engine speed SE is used at 116 to access a look-up table containing entries representing the IRL, which is also an empirically determined table. Then, at 118, the output of the look-up table is compared with the negative torque −TC commanded by the driver with the brake pedal as sensed from brake pedal position BC to determine the braking torque error TEB. The braking torque error signal TEB is then scaled by a value of KB through gain box 120 and used to affect the rate of change  of the ratio R after filtering at 112. It should be appreciated that the filtering in the brake torque control can be different if desired and that gain box 120 may contain additional filters.
As can be seen, therefore, FIG. 6 and FIG. 7 represent the controls for the configuration shown in FIG. 4 and, in principle, the controls for the configurations shown in FIG. 5 or other hybrid electric drive systems.
Consider typical operation shown in FIG. 7 in conjunction with the control diagram of FIG. 6. Assume that the vehicle is cruising at a fixed speed when the engine is supplying all the power to drive the vehicle and the electric motor/generator is supplying no power. Consider point A in FIG. 7 in this condition of steady state operation where PEP=0 and PC=PIOL is reached with the accelerator pedal position at ACA. If the driver suddenly depresses the pedal to a second position, which will be designated as ACB, meaning the driver wants to increase power, the torque increases instantly to point B along line L1 with torque supplied by the electric motor and battery. This is so because PEP is now greater than PIOL. Then TM is computed in block 114 if the vehicle is in the electric mode or block 126 if vehicle is in the hybrid mode. It will be appreciated that at this instant that =0. Then PC/SE supplies all necessary torque in electric mode and PC/SE−TIOLE or TC−TIOLE supplies all of the torque if in the hybrid mode. This motor torque signal is transmitted to block 102. The power desired by the driver is then achieved instantly. If the accelerator pedal is held constant at this point over time, then the torque of the electric motor will decrease along a line of constant power along line L2 in FIG. 7, thus holding the power constant as the vehicle accelerates. This line L2 represents the action of the feedback loop as designed in FIG. 6 which includes blocks 102, 104, 106, 108 and 110 (or 128 and 130), and 114 or 126. The vehicle will continue to accelerate with motor torque decreasing along line L2 until the point C is reached along the constant power line L2. This point is reached when PEP is iteratively reduced to zero and PC=PIOl. It will be appreciated that at all times during this process, the engine always operates along the IOL.
The car then will maintain this speed until the position of accelerator pedal is again changed. If the accelerator pedal is now reduced to the original position, the net torque will be reduced to point D, and speed will proceed back to point A along a constant power line L4. To accomplish this, the electric motor/generator must supply a negative torque to reach point D along line L3. This happens instantly. As the net torque and power proceeds along line L4, the electric motor/generator torque gradually approaches zero as the vehicle again begins to cruise when the accelerator position returns to ACA. Note that the deceleration maneuver returns energy to the battery system described above, and the acceleration maneuver takes energy from the battery system while the engine continues to operate along the IOL.
It will be appreciated, therefore, that the throttle opening of the engine is set to provide the best efficiency for a given power along the IOL. The electric motor is used to force the engine to operate along the IOL and to provide correct transient response to the vehicle. Note that a large electric motor and a small engine is preferred, but the invention can also employ a large engine and small electric motor with slower response. The CVT provides the correct speed and power setting as quickly as dynamics and motor capacity allow. The battery capacity is then used to temporarily provide and absorb energy to allow the CVT to change ratio without detrimental effects on performance. It will further be appreciated that this is accomplished, in the preferred embodiment, by having the engine and the electric motor on the same shaft in the preferred embodiment.
Based on the foregoing, it will be appreciated that the electric motor can be used to supplement and control the gasoline or diesel engine during both acceleration and deceleration of the vehicle, thus allowing the engine to run at optimum efficiency across its entire speed band with generally a fixed throttle setting or in an un-throttled state so as to maximize engine efficiency. This is not possible in a conventional continuously variable transmission system as discussed in FIG. 1.
Now, consider braking the vehicle with a brake command Bc in FIG. 6. As the brake pedal is depressed for a normal stop, switches SW1 and SW2 in FIG. 6 are set to the brake position. The braking level desired by the driver is compared with the ideal regeneration line (IRL) at block 118 at a given vehicle speed and transmission input speed ST or motor speed SM. The IRL is a line determined by testing the motor/generator and battery system for the best efficiency for energy storage at each speed. After such testing procedure, an ideal line can be selected to connect all the best efficiency points yielding the IRL.
The brake command Bc (at 34 in FIG. 6) represents a desired torque at the drive shaft or wheels of the car. At block 122 the torque command is divided by the ratio R to obtain the equivalent torque at the CVT input 124. This input is compared with the torque along the IRL at the speed of the motor SM at this instant. The error is used to command  through the gain block 120 and filter block 112. The ratio R of the transmission will change to seek the IRL via the feedback control system of blocks 102, 104, 106, 112, 116, 118 and 120. It is understood that this control system becomes ineffective when the ratio reaches its physical limits Rmin or Rmax.
The desired torque at the output of block 122 is sent to block 100 to compute the motor torque necessary to achieve the desired braking torque at the driveshaft and consequently the wheels of the car. Initially the torque at the motor is TC/R since R is zero at the start of the maneuver.
From the foregoing, it should be apparent that there is a need for systems and methods for efficiently and effectively controlling the rate of change of ratio , not simply the ratio, in a CVT. Furthermore, because a CVT is a drivetrain component and various load conditions can cause the CVT to slip, various approaches have been taken to control CVT pressure and minimize slip. However, conventional control mechanisms are mechanically based, using valves, orifices, and the like, and are conservatively designed for high pressure conditions which leads to lower efficiency and durability. Accordingly, there is also a need for a pressure control mechanism and method that controls the pressure in a CVT to prevent slip under all driver input conditions.